I would suggest you do not ...

I would suggest you do not choose the soak pit option as you could contaminate the groundwater.  Maybe consider an evaporation pond - a shallow pit that you will line with a suitable non-porous lining.  Look up the evaporation data vs rainfall for your area (i.e. net evaporation) for the season you need the temporary solution, and size the pond accordingly.  Depending on what the laboratory does, you may need to partially treat the waste, e.g. with lime, if there is a risk to birds or other wildlife.